If you have your trees in quality pots, or exhibition pots, obviously white paint is a very bad idea. White paint is okay for cheap terra cotta pots and plastic pots. Tape can leave gum residue on quality pots which could become a problem.
Greenhouse growers of potted plants,, will often have the pots in flats that keeps the pots tightly together. This way the pots shade each other. Several orchid growers I know leave a row of empty pots on the outside edge of the flats so that the empty pots shade the pots in use in the center.
Another trick, Nest a plastic pot inside a larger empty clay pot. The air space between the larger clay pot and the inner plastic pot with the plant will keep the plastic at ambient temp. The terra cotta clay will wick water and by evaporative cooling can be used to cool roots too if sprayed down occasionally on hot days. So inside a 5 inch diameter terra cotta pot set a 4 inch diameter plastic pot with the tree in the plastic pot.
